The present invention relates to an image forming device which forms an image on a recording medium by means of a toner jet, for use in a printing section of a digital copying device or facsimile device, or in a digital printer, plotter, etc.
Conventionally, many image forming devices have used the so-called electrophotographic process, in which image information is converted into light, which is projected onto a photosensitive body to form an electrostatic latent image thereon, which is then developed with toner. However, in recent years, in accompaniment with the rapid progress of digitalization due to the dissemination of computers, etc. and improvement in their performance, image forming devices of the jet type, which form an image directly using a jet of ink or toner, have been proposed, with the object of forming high-quality images using a simpler structure.
Various methods have been proposed for toner jet image forming devices, which form an image directly using a jet of toner, because they are capable of realizing high-quality, visually excellent printing using toner equivalent to that used in the electrophotographic process, and because they do not require an optical writing system, photosensitive body, etc.
For example, Japanese Examined Patent Publication No. 1-503221/1989 (Tokuhyohei 1-503221, published on Nov. 2, 1989) discloses a method of applying a voltage to wires provided in matrix form to form a charge image in the vicinity of the wires, and then allowing toner to act on the charge image to form a toner image.
Since printing is performed solely by a toner jet effect from a toner carrier to a transfer material, this kind of toner jet image forming device calls for stable control of the jet of toner particles and prevention of scattering of the toner particles after striking the transfer material.
For example, Japanese Unexamined Patent Publication No. 5-208520/1993 (Tokukaihei 5-208520, published on Aug. 20, 1993) discloses a method which attempts to prevent scattering of the toner particles when striking the transfer material after jet control, by using a capsule toner which, upon striking the transfer material, releases an adhesive member, causing the toner to adhere to the transfer material, and preventing secondary scattering of the toner.
Further, Japanese Patent Application No. 9-61488/1997 (Tokuganhei 9-61488), corresponding to Japanese Unexamined Patent Publication No. 10-250137/1998 (Tokukaihei 10-250137, published on Sep. 22, 1998), discloses a method which attempts to reduce the speed of the jet, in order to weaken the shock of collision of the toner particles with the transfer material, by applying a braking potential to reduce scattering.
Further, Japanese Patent Application No. 9-67000/1997 (Tokuganhei 9-67000), corresponding to Japanese Unexamined Patent Publication No. 10-258539/1998 (Tokukaihei 10-258539, published on Sep. 29, 1998), discloses a method which provides separation distance reducing means, which cause a magnetic field to act on charged developer particles (toner particles) which separate immediately before or after striking a recording medium, thus reducing the distance of separation thereof.
However, a problem with conventional toner jet image forming devices is that they are prone to so-called scattering, in which the high speed of collision of the toner with the transfer material causes the toner particles to rebound on impact, and scatter in the vicinity of the impact position, thus greatly impairing image quality.
In this connection, the method disclosed in Japanese Unexamined Patent Publication No. 5-208520 above prevents scattering of toner by means of a capsule toner which releases an adhesive member upon striking the transfer material, whereby the shock of collision when the toner particles strike the transfer material breaks the capsules to release the adhesive member contained therein. However, this method greatly restricts the materials which can be used for the toner, and it is difficult to obtain desired charging characteristics.
Further, in charging the toner, forces to which the toner particles are subjected must be constrained to an extent at which the toner particles will not break, thus restricting the charging methods which can be used to methods which do not involve excessive friction or pressure.
The method proposed in Japanese Patent Application No. 9-61488, on the other hand, uses conventional toner, and reduces the force of collision of the toner when striking the transfer material by forming an electrical field which reduces the speed of the toner jet. However, since the electrical field toward the transfer material is weak at the impact surface, rebound of and repulsive force among toner particles causes them to spread out horizontally.
Further, in the method of Japanese Patent Application No. 9-67000, in order to reduce scattering of the toner particles immediately before and after striking the recording medium, a magnetic field acts on the charged toner particles to create a force which causes the toner particles to converge. Thus a very strong magnetic field is necessary, and the size of the device is increased.
In light of the foregoing shortcomings with conventional toner jet image forming devices, there is a need for a method of reducing toner scattering by means of a simple structure.
It is an object of the present invention to provide a toner jet image forming device which is able to form good pixels and good images by reducing scattering which occurs when toner particles strike a transfer material.
In order to attain the foregoing object, an image forming device according to the present invention comprises: a toner carrier, which carries toner charged with a predetermined polarity; a counter electrode, provided opposite the toner carrier, to which a voltage can be applied; a control section, provided between the toner carrier and the counter electrode, having a plurality of gates, each forming a space for passage of a jet of toner from the toner carrier toward the counter electrode; and voltage applying means, capable of applying potentials independently to each of the gates of the control section; the image forming device controlling passage of toner through a given gate by application of a first potential which allows passage of the toner through that gate and a second potential which prevents passage of the toner through that gate, and forming a toner image on a transfer material, which is in contact with the counter electrode and which moves relative to the control section; in which a potential difference VH between the toner carrier and the counter electrode and an intensity EH of an electrical field in the vicinity of the counter electrode satisfy VHxe2x89xa61.5 kV and EHxe2x89xa71.8 kV/mm, respectively.
With the foregoing structure, by optimizing the potential difference between the toner carrier and the counter electrode and the electrical field intensity in the vicinity of the counter electrode (i.e., the electrical field intensity on the surface of the transfer material), good toner images with little toner scattering can be formed.
In other words, as shown in FIGS. 19(a) and 19(b), toner particles T leave the toner carrier, pass through the gates of the control section, and jet toward the counter electrode 11.
At this time, if the speed of the jet is high, repulsive force when the toner particles T strike the transfer material P is increased, and the toner particles T rebound greatly, as shown in FIG. 19(a), thus increasing scattering to areas surrounding the pixel.
If, on the other hand, electrical field intensity in the vicinity of the transfer material P is weak, the influence of repulsive force among the toner particles T, charging of the transfer material P, etc. causes the toner particles T to spread out over the surface of the transfer material P immediately before and after impact, as shown in FIG. 19(b).
Increasing the electrical field intensity and the distance the jet travels increases the speed of the toner jet, and thus increases rebound of the toner particles. However, decreasing the intensity of the jet electrical field increases the mutual repulsive force among the toner particles while in the jet and upon striking the transfer material, and also decreases the force with which they are attracted toward the counter electrode 11, thus causing spreading of the toner image. In the present invention, xe2x80x9cjet electrical fieldxe2x80x9d means an electrical field which causes the toner T to jet from the toner carrier toward the counter electrode 11 and strike the transfer material P, and is an electrical field intensity in the vicinity of the counter electrode 11 and the transfer material P (EH, E1).
As a result of assiduous investigations conducted to resolve the foregoing problems, the present inventors found that a converged toner image with little scattering can be formed if a potential difference VH between the toner carrier and the counter electrode 11 and an intensity EH of an electrical field in the vicinity of the counter electrode 11, which influence the speed of the toner jet when striking the transfer material, are set to VHxe2x89xa61.5 kV and EHxe2x89xa71.8 kV/mm, respectively.
Incidentally, if the control section is made up of a single layer of control electrodes only, the potential difference between the control electrodes and the counter electrode 11 and the electric field intensity in the vicinity of the counter electrode 11 vary according to the ON potential (first potential) and OFF potential (second potential) of the control electrodes, but if the foregoing conditions are satisfied, a converged toner image free of scattering can be obtained.
Further, in order to attain the foregoing object, an image forming device according to the present invention comprises: a toner carrier, which carries toner charged with a predetermined polarity; a counter electrode, provided opposite the toner carrier, to which a voltage can be applied; a control section, provided between the toner carrier and the counter electrode, having a plurality of gates, each forming a space for passage of a jet of toner from the toner carrier toward the counter electrode; and voltage applying means, capable of applying potentials independently to each gate of the control section; the image forming device controlling passage of toner through a given gate by application of a first potential which allows passage of the toner through that gate and a second potential which prevents passage of the toner through that gate, and forming a toner image on a transfer material, which is in contact with the counter electrode and which moves relative to the control section; in which the image forming device further comprises, between the control section and the counter electrode, a first electrode layer extending two-dimensionally opposite the counter electrode, provided with holes, corresponding to the gates of the control section, for passage of toner; and a potential difference VH between the toner carrier and the counter electrode, a potential difference V1 between the first electrode layer and the counter electrode, a distance d1 between the first electrode layer and the counter electrode, and an intensity E1 (=V1/d1) of an electrical field in the vicinity of the counter electrode satisfy VHxe2x89xa61.5 kV and E1xe2x89xa71.8 kV/mm, respectively.
In other words, the present inventors found that by providing the first electrode layer, through which toner can pass and to which a voltage can be applied, between the control section and the counter electrode, and by setting a potential difference VH between the toner carrier and the counter electrode, a potential difference V1 between the first electrode layer and the counter electrode, a distance d1 between the first electrode layer and the counter electrode, and an electrical field intensity E1 in the vicinity of the counter electrode (=V1/d1) (which are factors which influence toner jet speed when striking the transfer material) so as to satisfy VHxe2x89xa61.5 kV and E1xe2x89xa71.8 kV/mm, regardless of the distance of the control section from the counter electrode, the electrical field intensity E1 could be optimized, and a converged toner image with little scattering could be formed.
For example, since the distance between the toner carrier and the control section is typically maintained at around 100 xcexcm, for example, the control section is usually integrally provided with a toner supply section, which includes the toner carrier. In such a case, rotation of the toner carrier may cause vibration of the control section in the direction of the toner jet. However, in the present invention, since the conditions of control of the toner when it strikes the transfer material are determined by the voltage applied to the first electrode layer and the distance between the first electrode layer and the counter electrode, a converged toner image with little scattering can be formed even if such vibration occurs.
Incidentally, instead of providing the first electrode layer separately from the control section, it may be provided integrally therewith.
Further, the first electrode layer may be provided so as to come into contact with the transfer material at a position upstream, with respect to the direction of movement of the transfer material, from a position at which the toner strikes the transfer material, so that the first electrode layer is a fixed distance from the counter electrode and the transfer material. In this way, it is possible to maintain a constant interval between the first electrode layer and the transfer material.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.